During the screening of pulp suspensions, high pulp concentrations, for example, from 3 to 5%, are desired in order to achieve high production capacity and to avoid the transporting of unnecessarily high liquid volumes through the screening system. The use of high concentrations, however, creates considerable difficulty in then separating the undesirable fractions from the pulp. That is, the apertures in the screen plate being utilized can easily become clogged, and it becomes difficult to selectively separate the impurities at low reject withdrawal rates. These difficulties are primarily a result of the thickening of the reject portion, which itself takes place because the liquid preferentially follows along with the accept fraction through the screen plate. This problem is avoided in conventional screens by the reject portion being diluted by the addition of further liquid. This is undesirable, however, for the reasons set forth above.
Various screen designs have thus been developed in an attempt to solve these problems. One example of such a screen design is the arrangement of wing sections on a rotary member, which is to be moved along the screen member in order to bring about instantaneous cleaning pulses, and to thus prevent the clogging of the screen apertures. Such a design is shown, for example, in U.S. Pat. No. 4,328,096. The problem of reject thickening, however, is not solved by such a device, which is also still not applicable to high pulp concentrations.
In European Patent Application No. 206,975, for example, another screening device is shown, in this case comprising a screening cylinder and an inner rotor, which is provided with members which are intended to bring about pulsations in the pulp suspension. These members have a cross leading edge, and a curved surface therebehind, the distance of which from the screening cylinder increases successively. The leading edge thus produces a positive pressure pulse, and the curved surface produces a negative pressure pulse, in order to thereby bring about a separation of impurities over the screen plate. By using this design, however, there is a risk that the pulp will be transported about by the cross leading edge to too great an extent, thus decreasing the relative speed between the rotor and the pulp, until the suction pulse ceases and the screening process is terminated. The screen thus becomes blind, this effect decreases, and the accept flow itself ceases. Moreover, the cross leading edge yields a short, strong pressure pulse, which has a negative effect on the cleaning process. A similar design is shown in U.S. Pat. No. 4,200,537. According to the device shown in this patent, the rotor can be arranged to rotate in different directions. The embodiment shown in FIG. 3 of this patent corresponds to the aforesaid European patent publication, and has the aforementioned disadvantages. The embodiment shown in FIG. 2 therefore provides a sloping leading surface, and a cross trailing edge of the pulsation members. This gives rise to problems with the thickening of the reject, again in the manner stated above.